1. Field of the Invention
The present invention relates to an image sensor, more particularly, to an image sensor having an electronic shutter that does not use a charge coupled device (CCD).
2. Description of Related Art
Currently, there are two types of image sensors, a charge coupled device (CCD) image sensor and a MOS image sensor array.
In the CCD image sensor comprising a plurality of pixels, charges generated by sensing light of each of the pixels are transmitted to and stored in a charge coupled device (CCD), and then the stored charges are output from a terminal of the CCD in sequence. If the charges induced by the respective pixels are transmitted into the CCD simultaneously, an effect as an xe2x80x9celectronic shutterxe2x80x9d is achieved, so that a proper signal can be obtained at a proper time.
The MOS image sensor array does not use a CCD. With reference to FIG. 1, which shows a 4xc3x973 image sensor array, the array comprises 12 pixels (90) arranged regularly, four word lines (93), three bit lines (94), a horizontal scanning circuit (95), a vertical scanning circuit (96), a crossing circuit (97), and an output terminal (98). Each of the pixels (90) comprises a photosensitive diode (91) and a control transistor (92). Each of the word lines (93) is connected with three pixels (90) in a row, respectively. Each of the bit lines (94) is connected with four pixels (90) in a column. The horizontal scanning circuit (95) and the vertical scanning circuit (96) respectively scan the photosensitive diodes (91) in sequence. The photosensitive diode 91 in each of the pixels (90) senses light to generate induced charges, and then the induced charges are fed to one of the corresponding bit lines (94). Then the charges induced by each of the pixels (90) are output via the output terminal (98) to an external circuit (not shown) in sequence, so that the charges can be converted into image signals.
Although the MOS image sensor array is easy to manufacture, and has the advantages of low power consumption and low cost, it cannot function as an electronic shutter as the CCD image sensor does.
The control transistor (92) of each of the pixels (90) simply transfers the charges from the photosensitive diode (91) to the corresponding bit line (94). The control transistor (92) does not store the charges. During the turn-on status of the control transistor (92) of one of the pixels (90), the control transistor. (92) transmits the charges of the photosensitive diode (91), while the other pixels (90) successively sense the variance of the external light. Accordingly, if a proper image is to be obtained, the pixels (90) have to sense light in sequence, and the induced charges in the respective pixels (90) have to be read in sequence, so that the duration for sensing light by the respective pixels 90 is identical. Therefore, the MOS image sensor array is appropriate only for a static image or a moving image varying very slowly, such as the image of a monitoring camera. That is, the conventional MOS image sensor array is not suitable for a dynamic or rapidly varying image.
One object of the present invention is to provide an improved image sensor having an electronic shutter function.
Another object of the present invention is to provide an image sensor having a simple structure without using a CCD.
In accordance with one aspect of the present invention, the image sensor comprises a plurality of pixels, each of which has a photo storage cell, a control switch connected with one terminal of the photo storage cell, and a reset switch connected with the other terminal of the photo storage cell. Each of the pixels of the image sensor controls the switches to sense light to induce charges and store the induced charges. Accordingly, the image sensor can achieve an electronic shutter effect.
In accordance with another aspect of the present invention, the photo storage cell of each pixel of the image sensor comprises an upper layer, which is an electrode plate, and a lower layer, which is made of silicon.
In accordance with another aspect of the present invention, the electrode plate layer of the photo storage cell is connected to a bit line via a sample-and-hold switch, while the silicon layer thereof is connected to a reference voltage via a reset switch. Operation of the photo storage cell is controlled by the two switches, so that the photo storage cell can perform operations such as resetting, light sensing, charge storing and charge reading, in sequence.
In accordance with still another aspect of the present invention, the photo storage cell of each pixel of the image sensor is a MOS transistor with a transparent gate.
In accordance with still another aspect of the present invention, the photo storage cell is an NMOS or a PMOS transistor with a transparent gate.
In accordance with still another aspect of the present invention, the photo storage cell comprises a photosensitive diode and a capacitor connected with each other in series.
In accordance with still another aspect of the present invention, the photo storage cell comprises a photosensitive diode and another diode connected with each other in series.